Activated carbon filter and process for the separation of noxious gases

ABSTRACT

Activated carbon filters containing transition metals, are prepared by (a) exchanging the transition metal with a cellulose ion exchange material to produce a cellulose material containing the transition metal; (b) charring of the product of (a); activating the product of (b) to form an activated carbon filter having a pore network throughout; and removing surface carbon substantially throughout the pore network of the filter formed in (c). Methods of filtering an atmosphere containing a gaseous contaminant by passing the contaminated atmosphere through a filter made by this process are described.

The present invention relates to filtration processes. It isparticularly concerned with the use of activated carbon filters toremove toxic chemicals from a breathable atmosphere.

According to the present invention a method of filtering an atmospherecontaining a gaseous contaminant comprising passing said atmospherethrough an activated carbon filter containing at least one transitionmetal, the filter material having been prepared in a process including,inter alia, the exchange of the metal ion with a cellulose ion exchangeresin.

Preferably the transition metal is copper, cobalt, chromium or silver. Asuitable process for the production of the activated carbon filtercontaining one or more of these metals is described by P A Barnes and EA Dawson in “A New Method for the Production of Metal-Carbon Catalysts”published in the Proceedings of the 6th International Symposium onCatalyst Preparation, University of Louvain-la-Neuve, September 1994. Ingeneral terms in this process the starting material is typically an ionexchange material in the form of carboxymethyl cellulose (Whatman CM32)as an alkali metal salt such as a sodium salt. Hydroxyl groups on thecellulose chain are modified to form an ether group which carries ametal carboxylic substituent, for example of formula O(CH₂)₂COOM where nis an integer of from 1 to 6 and M is an exchangeable cation. Aparticular group is OCH₂COONa, with sodium as an exchangeable cation. Anion exchange reaction is set up with a suitable metal salt, preferably anitrate or sulfate of the metal, for example copper sulfate, and theresulting residue is dried and then charred in an inert gas flow. It isthen cooled under an inert gas and this is followed by activation in anitrogen stream containing steam, to result in a carbon matrix holdingthe metal relatively uniformly dispersed throughout. This primaryactivation may be followed by a secondary oxidation by heating in a flowof oxygen in helium to chemisorb oxygen on the carbon surface. Theresulting pore widening improves access to the metal by the gases beingfiltered.

The amount of metal present is preferably arranged to be between 3% and18% by weight.

Preferably the metal is copper, but cobalt and silver are alsoeffective, singly or in combinations with one another or copper. Whensuch combinations are contemplated the ion exchange process for each maytake place simultaneously. The percentage ion exchange and the carbonactivation time both have significant effect on the property of theresultant filter to adsorb hydrogen cyanide. Low ion exchange, less than50% and preferably about 25% or less has been found to favor dispersionof the metal and to increase the capacity of the fitter to adsorb HCN.Long activation times, for example 6 to 12 hours or more increase thecapacity to adsorb HCN very considerably. The activation is catalyzed bythe presence of copper, and the metal then becomes a center for evolvedgases and in the final product the nucleus for transport passagesthrough which reacting gases may diffuse. Cobalt and silver are alsoboth capable of this catalytic effect.

The reaction of hydrogen cyanide with copper and copper salts onactivated carbon results in the release of cyanogen (CN)₂ as a volatilereaction product. Due to the toxic nature of cyanogen additionalmeasures may need to be taken for its removal, particularly when copperis the primary metal present. This is much less the case when cobalt isthe primary metal.

The filter material may be arranged to contain chromium, in addition tothe copper, cobalt or silver. Where copper and chromium are employed,the presence also of silver is particularly useful. There is no lowerlimit for the secondary metal. The upper limit may be of the order of11% by weight.

Particularly good results in terms of the removal of both hydrogencyanide and the product cyanogen are achieved in activated carbonfilters containing copper, chromium and silver. Various examples offiltration and the construction of filters suitable for use in processesaccording to the invention will now be described, by way of example,with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an ion exchange process,

FIG. 2 is a graph comparing water adsorption properties, and

FIG. 3 is a graph comparing water desorption properties.

Table 1below lists a number of samples of filter materials which weretested for their ability to remove HCN.

DETAILED DESCRIPTION OF THE INVENTION

TABLE 1 Exchanged Percentage Number of HCN Pulses before Sample metalion metal by weight pulses removed cyanogen breakthrough 502 Cu²⁺ 2 6 —505 Cr₂O₇ ²⁻ 13 8 — 506 Ag²⁺ 5 60 — 507 Cu²⁺ 10 80 — 5070 Cu²⁺ 10 120 —508 Cu²⁺ 17 46 — 510 Cu²⁺/Cr²⁺ 8/9 98 — 511 Co²⁺ 16 85 62 512 Cu²⁺/Co²⁺8/8 53 60 513 Cu²⁺/Cr³⁺/Ag+ 5/1/3.5 141 120  515 Cu²⁺/Ag+ 5/1 25 12 517Ag⁺ 18 15 — 518 Cu²⁺/Co²⁺/Ag+ 6/—/1 54 40 ASC Cu/Cr/Ag 6-9/1.5-3.5/0-0.5120 85 (whetlerite)

Samples 506 to 518 represent examples of filter materials in accordancewith the invention, while samples 502, 505 and ASC are outside the scopeof the invention and are included for the purposes of comparison.

Thus sample 506 is a filter carbon containing 5% by weight silver, 507contains 10% by weight copper etc. These samples 506 to 518 wereprepared as now described below. The starting material for the creationof filter carbon was carboxymethyl cellulose (Whatman, CM32), this beinga sodium salt ion exchange medium. Hydroxyl groups on the cellulosechain were modified to OCH₂COONa, with sodium as the exchangeablecation. Ion exchange was then conducted using copper sulfate, cobaltnitrate and silver nitrate to give the desired metal content, the ionexchange process being as illustrated in FIG. 1.

The exchange process was conducted at room temperature by stirring themetal salt solution into a mixture of 1 gm cellulose per 100 cm³ water,then leaving to stand for several hours. Afterwards the solid wasfiltered off, washed thoroughly in deionized water and dried. The driedmaterial was then charred at 400° C. for one hour under a nitrogen flow.After cooling under nitrogen the carbon was activated by heating to 600°C. for 2 hours in an atmosphere of flowing nitrogen saturated with watervapour at 25° C. A relatively low temperature could be used for theactivation because of the catalytic effect of the metal on carbongasification, and one advantage of this low temperature was theminimization of sintering of surface metal particles. The standardactivation time was 2 hours.

This then was the process employed for the production of samples 506 to518. Sample 5070 however was then subjected to a further oxidation byheating to 150° C. in a flow of 5% oxygen in helium to chemisorb oxygenon the carbon surface. Removal of surface carbon as CO₂ was thenachieved by heating to 350° C. in a pure helium flow. The oxygen/heliumcycling procedure was repeated four times to give a controlled stepwiseremoval of carbon throughout the pore network. This results in a carbonstructure in which there is, so to speak, a lattice of interconnectingpores, with the metal trapped at the pore junctions and uniformlydistributed throughout the carbon.

Table 2 below shows the effect of activation time and percentage ionexchange upon the total and copper surface areas, and upon HCNadsorption, of various samples of filter material according to theinvention, all of which comprised only copper in carbon.

TABLE 2 Total Activation % ion surface HCN pulses Sample time (h)exchange % copper area Dispersion Cu area removed 521 2 100 10.1 372 4.03.8 9 522 4 100 10.6 344 14.2 14.3 17 523 6 100 10.2 437 16.5 16 25 5242 50 6.4 449 15.2 9.2 20 525 4 50 6.5 443 26.1 16.1 29 526 6 50 6.5 49735.5 21.9 47 527 2 25 3.4 389 13.3 4.3 19 528 4 25 3.4 359 9.3 3.0 28529 6 25 3.5 468 17.5 5.8 57 530 12 25 3.5 518 67.8 22.5 110

The carbons were tested against pulses of 10,000 mg/m³ HCN/air mixture,the air having been at a relative humidity of 80%. Prior to testing thecarbons were sieved, with the fractions between 600 μm and 150 μm beingused. Carbon samples (10 mg) were loaded into glass tubes 60 mm inlength and 2 mm in internal diameter with a glass fiber plug at eachend. Each sample was then compressed to the same degree by applying aweight to the carbon. The glass tubes were then loaded into a ChrompackCP9001 packed gas chromatography oven at 140° C. with a nitrogen carrierflow of 10 ml/min and a head pressure of 175 KPA. The effluent from thesample tubes was split; 0.75 ml/min being transferred by capillary clmto a mass spectrometer and 9.25 ml/min passing through a flameionization detector (FID). Once the baseline from the FID had settledthe samples were pulsed with the HCN vapour using an automated gassampling valve with a 1 ml sample loop. Pulses were supplied everyminute until breakthrough was observed.

It is clear from the Tables that carbon filter materials formed withcopper, cobalt or silver introduced by an ion exchange mechanism asabove described perform significantly well in the removal of HCN from anatmosphere. Those outside the preferred range performed much less well.The most significant results were obtained with sample 513 whichcontained chromium and silver as secondary metals. This showed a veryhigh HCN adsorption coupled with a high retention of (CN)₂.

Sample 513 was deliberately created to compare with the sample labeledASC. ASC was a conventional coal-based carbon impregnated with anammoniacal solution of copper, chromium and silver, so that the finalproduct contained the metals as salts on the carbon surface. Sample 513performed significantly well in the adsorption of hydrogen cyanide andin withholding cyanogen. The ASC sample however, whilst exhibiting quitegood hydrogen cyanide adsorption, demonstrated an extremely sharpcyanogen breakthrough which resulted in detector overload.

The results in Table 2 show that for samples containing the same amountof copper, increase in activation time caused an increase in the totalLangmuir m²/g surface area. Sample 530 demonstrates in particular howimportant an effect activation time can have upon hydrogen cyanideabsorbability. The Table demonstrates that while an activation time of 6hours produces a significant improvement over lower times, 12 hoursgives particularly good results.

It is also discernible from Table 2 in comparison with Table 1, that thelower percentage ion exchange of 25% gave better results than 50% andcertainly than 100%.

FIGS. 2 and 3 show that there was no great difference in wateradsorption and desorption between the carbon filter materials accordingto the invention and SCII and ASC carbon at levels of P/P₀ below 0.5.Above that level however carbon materials in accordance with theinvention performed significantly better.

What is claimed is:
 1. A method of manufacture of an activated carbonfilter comprising at least one transition metal, the method comprising:(a) exchanging of at least one transition metal with a cellulose ionexchange material to produce a cellulose material comprising the atleast one transition metal; (b) charring of the product of (a); (c)activating the product of (b) to form an activated carbon filter havinga pore network throughout; and (d) removing surface carbon substantiallythroughout the pore network of the filter formed in (c).
 2. A methodaccording to claim 1 wherein step (d) is performed at least twice.
 3. Amethod according to claim 1 wherein the at least one transition metal iscopper, cobalt, chromium or silver.
 4. A method according to any claim 1wherein the cellulose ion exchange material is an alkali metal saltcarboxymethyl cellulose.
 5. A method according to claim 1 wherein thecellulose ion exchange material carries an ether group having theformula —O(CH₂)_(n)COOM where n is an integer from 1 to 6 and M is anexchangeable cation.
 6. A method according to claim 1 wherein thecellulose ion exchange material carries an ether group having theformula —O(CH₂)_(n)COOM where n is an integer from 1 to 6 and M is anexchangeable cation, in which the ether group is formed by modifyinghydroxyl groups on the cellulose chain of a precursor.
 7. A methodaccording to claim 1 wherein step (a) comprises exposing of thecellulose ion exchange resin to a solution of a salt of the at least onetransition metal.
 8. A method according to claim 1 wherein step (b)comprises heating the product of step (a) in an inert atmosphere.
 9. Amethod according to claim 1 wherein step (c) comprises heating theproduct of step (b) in a flowing nitrogen atmosphere which is saturatedwith water vapor at 25° C.
 10. A method according to claim 1 whereinstep (c) comprises heating the product of step (b) in a flowing nitrogenatmosphere which is saturated with water vapor at 25° C. and furtherwherein the product of step (b) is heated to 600° C.
 11. A methodaccording to claim 1 wherein step (d) comprises the sequential steps ofoxidizing the surface carbon and thereafter removing the oxidizedsurface carbon as carbon dioxide.
 12. A method according to claim 1wherein step (d) comprises the sequential steps of oxidizing the surfacecarbon and removing the oxidized surface carbon as carbon dioxide andwherein the step of oxidizing the surface carbon comprises heating in anatmosphere comprising oxygen.
 13. A method according to claim 1 whereinstep (d) comprises the sequential steps of oxidizing the surface carbonand removing the oxidized surface carbon as carbon dioxide and furtherwherein the step of oxidizing the surface carbon comprises heating to150° C. in an atmosphere comprising 5% vol./vol. O₂ in helium.
 14. Amethod according to claim 1 wherein step (d) comprises the sequentialsteps of oxidizing the surface carbon and removing the oxidized surfacecarbon as carbon dioxide, further wherein removing the oxidized surfacecarbon as carbon dioxide comprises heating in an inert atmosphere.
 15. Amethod according to claim 1 wherein step (d) comprises the sequentialsteps of oxidizing the surface carbon and removing the oxidized surfacecarbon as carbon dioxide and wherein removing the oxidized surfacecarbon as carbon dioxide comprises heating to 350° C. in an atmosphereof helium.
 16. An activated carbon filter produced by the method ofclaim
 1. 17. An activated carbon filter produced by the method of claim1, wherein the amount of metal present is between 3% and 18% by weight.18. An activated carbon filter produced by the method of claim 1,wherein the amount of chromium present is up to 11% by weight.
 19. Anactivated carbon filter containing at least one transition metal,wherein the carbon filter has a structure in which there is a lattice ofinterconnecting pores, said at least one transition metal being trappedat pore junctions and is substantially uniformly distributed throughoutthe carbon, and wherein the filter material has been prepared in aprocess including the exchange of at least one transition metal with acellulose ion exchange resin and the removal of surface carbon from thepore network of the filter.
 20. A method of filtering an atmospherecontaining a gaseous contaminant comprising passing said atmospherethrough an activated carbon filter of claim
 19. 21. A method offiltering an atmosphere containing a gaseous contaminant comprisingpassing said atmosphere through an activated carbon filter of claim 19.